In U.S. Pat. No. 2,905,756 issued to R. E. Graham on Sept. 22, 1959, a method and apparatus for reducing the bandwidth needed to transmit a television or other pictorial information signal is disclosed wherein the difference between the original signal and a prediction thereof is quantized and encoded. The prediction is formed adaptively by analyzing each of several possible predictions in light of previously encoded data and selecting the one prediction that should produce the best result. Each of the possible predictions is also formed by reference to previously encoded information available at both the transmitter and receiver, thus eliminating any need to encode predictor selection data.
Adaptive or switched predictor differential pulse code modulation (DPCM) systems of the type suggested by Graham are extremely sensitive to errors which may occur during transmission of the encoded information between the transmitter and receiver, since the effect of any transmission error is cumulative and propagates so as to contaminate all succeeding information until correction is achieved through additional processing logic. Error accumulation occurs because only the differences between each prediction and the original signal are encoded, and the pictorial information is decoded by summing all of the differences in an integrator or adder. To reduce propagation of transmission errors, some prior art systems incorporate check characters in the encoded data, and upon detection of an error, use various correlation techniques to locate the point of occurrence of the error and correct it. Because redundant information must be added to the signal, some additional bandwidth is required in the transmission medium and the receiver must be outfitted with error detection and correction logic, which can be costly.
In another approach designed to avoid the propagation of transmission errors, it has been found advantageous to intentionally introduce an attenuation or "leak" in the predictors used in both the encoder and decoder so that the predictions statistically tend to converge toward a preselected constant value. The technique works well for certain fixed predictor DPCM systems because the error in an erroneously decoded picture element eventually decays to zero in future picture elements instead of propagating indefinitely. The technique does not work as well in adaptive predictor DPCM systems, because the erroneously decoded intensity value may also affect the predictor selection rule. In this event, the decoder and the encoder use different rules to process future picture elements causing distortion in the received picture.
In view of the foregoing, it is the broad object of the present invention to improve the encoding of pictorial information so as to reduce the propagation of errors that may occur in the transmission medium between transmitter and receiver. Specific objects are to reduce error propagation in an adaptively predicted DPCM system without undue distortion of the reconstructed picture and without adding significantly to the bandwidth required for transmission or the logic needed for error detection and correction.